You have a slight form of diarrhea or hemorrhoids. When you pass waste, sometimes a little stays just inside if you've got diarrhea, and it'll seep out. That can cause terrible irritation. Also, sometimes a person can have a hemorrhoid, which is swollen tissue that comes from straining whilst passing waste. To sooth your behind for either condition, put PLAIN KY jelly on there from the drugstore, use it daily after you shower until this stops, and the lubrication will keep the skin from itching so much when any seepage happens. You can also take baby wipes around with you for when it itches, to clear excess waste when it happens and soothe the area.
To prevent this from happening so much, there are three things to do for good digestion: Plenty of exercise, plenty of water, and plenty of fiber foods (like salads, whole wheat bread). In addition, you could drink some Acidophilus milk or eat Activia yogurt for a couple weeks, it has "good" bacteria in there that helps break down waste better, so you might not have so much remnant waste seeping out so much.
Digestive issues like this are very common, we eat such random diets, we don't drink enough water... rather we drink sodas, we sit around a lot, so all this leads to indigestion of all sorts.